1. Field of the Invention
The invention relates to a vehicle having at least one electric machine that can be operated as a generator. In this context, the term vehicle refers to a non-track-bound vehicle, and particularly a road vehicle.
2. Description of the Related Art
Vehicles, such as hybrid vehicles or electric vehicles, have one or more electric machines with different operating modes that function in view of respective operating situations of the vehicle. For example, at least one electric machine can be operated as a motor to provide at least additional drive for the vehicle. Full hybrid vehicles and electric vehicles are driven exclusively, at least for a certain time, by the electric machine. However, the electric machine of mild hybrid vehicles only provide a supplementary drive for the vehicle in addition to an internal combustion engine. A supply of electrical energy from a vehicle-mounted electrical energy accumulator is provided when an electric machine is operated as a motor. The electrical energy accumulator may be a lithium-ion high voltage battery. The drive power achieved by operating the electric machine as a motor depends on the size of the electric machine. For example, good drive power is necessary to drive a heavy vehicle, and as a result heavy electric machines have been considered necessary for a heavy vehicle. WO 2007/125743 proposes that a motor for driving a vehicle should be operated at overload to avoid using a heavy electric machine as a motor. However, an overload can damage the electric machine, and complex means for monitoring the temperature or time period of the motor are necessary in the overload mode.
An electric machine also customarily must be operated as a generator in hybrid or electric vehicles in addition to being operated as a motor. In this context, serial hybrid drives or power-split hybrid drives have drive arrangements with separate electric machines for the operating mode as a generator or as a motor. Parallel hybrid drives provide drive arrangements in which the same electric machine is operated either as a motor or as a generator. An electric machine must be operated if the electrical energy accumulator no longer contains sufficient electrical energy to supply electrical energy to the motor to drive the vehicle. In such cases, the internal combustion engine of a hybrid engine is activated to drive the vehicle and/or to operate the generator to charge the electrical energy accumulator with electrical energy.
An electric machine frequently is operated as a generator to recover kinetic energy from the traveling vehicle. As a result of the generator load which is applied when the electric machine is operating as a generator, the vehicle is braked and the resulting energy is converted at least partially into electrical energy. This electrical energy then is stored in the electrical energy accumulator and can be used again, for example, to drive the vehicle. Therefore, an electric machine is operated as a generator without the internal combustion engine having to be activated during recovery in a hybrid vehicle. To recover the kinetic energy of the vehicle as well as possible, the driver of the vehicle implements a deceleration request and at least one electric machine operates as a generator if this requested deceleration can be made available by the generator load that comes about. The service brake of the vehicle is not activated, and kinetic energy of the vehicle is converted into heat, until the requested deceleration is greater than the deceleration that can be made available by operating the electric machine as a generator. A vehicle with these features is described generally in DE 10 2006 034 936.
An object of the invention is to improve the vehicle disclosed generically in DE 10 2006 034 936.